Hamfest buy: space charge tube car radio

[wa2ise]

Picked up for $2 a Delco car radio meant for a 1960 Oldsmobile. model 989276. Uses space charge tubes, and a doorkbob transistor output. And it actually works, didn't expect that when I first powered it up. The doorknob does get rather warm, but looking at the huge heatsink it looks to be operating as designed.

[Olorin67]

With just a single output transistor, it needs to run class A, much like a single-ended tube amp, hence the large heatsink. Transistors were expensive back then, but it saved having an unreliable vibrator power supply, and an output transformer. Low power tubes could run off 12V, but not audio outputs. Similarly, in the 60's many makers of consoles put in transistor output stages , even if the rest of the unit still ran on tubes. Saved a lot of iron, and without output tube heaters, they saved on the power transformer too.

[old_coot88]

The failure rate of those output transistors was on a par with OZ4s and vibrators in the older all-tube radios. There was always a fractional-ohm resistor/fuse in the emitter leg of the transistor. It popped whenever the transistor shorted. We had to keep a running stock of those (something like .27, .33, .47 ohm IIRC). Don't recall ever having to replace one of the space-charge tubes, though.

[Olorin67]

I'm sure they didnt last if the speaker wires got shorted somewhere.

[wa2ise]

Originally Posted by Olorin67

... if the speaker wires got shorted somewhere.

I've heard that the bigger problem was if the speaker was disconnected. These radios used an output autoformer to drive the speaker, and like tube amps, no load conditions would allow excessive voltage spikes to happen. Which could take out the transistor. This radio has a shorting switch that is asserted when there is no connector on the speaker output terminal.

[dieseljeep]

Originally Posted by old_coot88

The failure rate of those output transistors was on a par with OZ4s and vibrators in the older all-tube radios. There was always a fractional-ohm resistor/fuse in the emitter leg of the transistor. It popped whenever the transistor shorted. We had to keep a running stock of those (something like .27, .33, .47 ohm IIRC). Don't recall ever having to replace one of the space-charge tubes, though.

I found open heaters in the space charge tubes. That's all that ever went wrong with the Motorola built sets.
When a Bendix built radio went bad in a 57-59 Ford, I would go to the auto salvage yard and buy a Motorola built radio. The customer was happier and better off for it.